Lakhmir Singh Science Class 8 Solutions For Chapter 6 .

Lakhmir Singh Science Class 8 Solutions For Chapter 6Combustion and Flame1. Define the ignition temperature of a substance.uteAnswerThe lowest temperature at which a combustible substance catches fire when heated in the air iscalled its ignition temperature of a substance.2. Which of the two has a lower ignition temperature: petrol or kerosene?Petrol has a lower ignition temperature.3. Name the most common fire extinguisher.AnswerInsThe most common fire extinguisher is water.titAnswer4. Which is the best fire extinguisher for fires involving electrical equipment and inflammablematerials like petrol?AnswerThe best fire extinguisher for fires involving electrical equipment and inflammable materials likepetrol is carbon dioxide.Answersh5. Name one substance which undergoes spontaneous combustion (or burns in air at roomtemperature).White phosphorous undergoes spontaneous combustion (or burns in air at room temperature).ka6. Name the unit in which the calorific value is expressed.AnswerCalorific value is expressed in energy per unit mass of the substance. Example KJ/KgAa7. Which of the following fuels has the lowest calorific value?Diesel, Methane, Coal, CNG, PetrolAnswerCoal has the lowest calorific value.8. Which of the following fuels has the highest calorific value?Diesel, Methane, CNG, Coal, PetrolAnswerCNG and Methane have the highest calorific value.

9. Name the term which is used to express the efficiency of a fuel.AnswerCalorific value is used to express the efficiency of a fuel.10. Name one solid, one liquid and one gas which burn by producing a flame.AnswerSolid: Molten waxuteLiquid: KeroseneGas: Liquified Petroleum Gas11. Which of the following does not produces a flame on burning?AnswerCharcoal does not produce flame on burning.titCamphor, Charcoal, Kerosene12. Name one fuel which burns without producing a flame.InsAnswerCharcoal burns without producing flame.13. How many zones are there in a flame?AnswerThere are three zones in a flame. Namely, innermost, middle and outer zone.Answersh14. Which zone of a candle flame is the hottest?The outermost zone of the candle is the hottest.ka15. In a candle flame, what is the colour of: (a) innermost zone (b) middle zone and (c) outerzone?Answer(a) Dark or blackAa(b) Yellow(c) Blue16. Name any harmful product released by the burning of fuels.AnswerCarbon monoxide is one of the most harmful products released by the burning of fuels.17. Name the very poisonous gas produced by the incomplete combustion of fuels.AnswerIncomplete combustion of fuels produces poisonous gas called carbon monoxide.

18. Name the fuel which is gradually replacing petrol and diesel in automobiles.AnswerCNG is gradually replacing petrol and diesel in automobiles.19. Name two substance having low ignition temperature and two having high ignitiontemperatures.Substances having low ignition temperature: LPG and PetrolSubstances having high ignition temperature: Coal and wood20. Fill in the blanks with suitable wordsuteAnswer(a) A fuel must be heated to its before it starts burning.(b)The most common supporter of combustion around us is(d)A liquid fuel used in homes istit(c)Fire produced by burning oil cannot be controlled byIns(e)The amount of heat evolved when 1kg of a fuel is burnt completely is called its(f)The substance which vaporize during, give(g)Burning of wood and coal causes of air.Answer(a) A fuel must be heated to its ignition temperature before it starts burning.sh(b)The most common supporter of combustion around us is air.(c)Fire produced by burning oil cannot be controlled by water.(d)A liquid fuel used in homes is kerosene.(e)The amount of heat evolved when 1kg of a fuel is burnt completely is called its calorific value.ka(f)The substance which vaporizes during, give flames.(g)Burning of wood and coal causes pollution of air.Short Answer Type QuestionsAa21. (a) What are fuels? Name any two common fuel.(b) State any four characteristics of an ideal fuel (or good fuel).Answer(a) Any substance which upon combustion produces a usable amount of energy is known as fuel.For example, fossil fuels, biogas, nuclear energy etc.(b) Characteristics of an ideal fuel (or good fuel) are: Fuel is easily available and cheaper Burns easily in the air at a moderate rate

Releases a large amount of energy No harmful substances must be emitted from fuelEco friendly22. (a) Define the calorific value of a fuel. (b) “The calorific value of LPG is 55000KJ/kg”. What does it mean?Answerute(a) The amount of heat energy produced on complete combustion of 1 kg of a fuel is called itscalorific value. The calorific value of a fuel is expressed in a unit called kilojoule per kg (kJ/kg).(b) The calorific value of LPG is 55000KJ/kg means 1 kilogram of LPG is burned completely and55000KJ/kg of heat energy is produced.23. Can you burn a piece of wood by bringing a lighted matchstick near it? Explain.titAnswerInsNo we cannot burn a piece of wood by bringing a lighted matchstick near it. When a lightedmatchstick is brought near a piece of wood it does not start burning because the ignition temperatureof wood is higher than a piece of paper. A matchstick can light a tiny splinter of wood but not a biglog of wood. A burning matchstick can produce sufficient heat to reach the ignition temperature ofthe splinter of wood therefore a matchstick can light a splinter of wood directly.24. Why do you have to use paper or kerosene oil to start fire in wood or coal?AnswerThe ignition temperature of wood or coal is higher and it requires more heat to start burning. This isthe cause that to start a fire in wood or coal paper or kerosene oil is used as the ignition temperatureof paper or kerosene oil is lower than that of wood or coal.Answersh25. What is meant by rapid combustion? Give one example of rapid combustion.kaWhen a substance burns rapidly, with the aid of an external source and produces heat within a shortperiod of time is called rapid combustion. Example: Burning of LPG26. What is meant by spontaneous combustion? Give one example of spontaneouscombustion.AnswerAaThe types of combustion in which material suddenly bursts into flames, without the application of anyapparent cause is called spontaneous combustion. Eg., Burning of phosphorus.27. What is meant by explosive combustion (or explosion)? Give one example of explosivecombustion (or explosion).AnswerA sudden reaction takes place with the release of heat and light and the evolution of a large amountof gas takes place it is called an explosion. Eg., firecrackers.28. How will you show that air is necessary for combustion?Answer

Air helps in combustion. Air contains about twenty-one percent of oxygen and this oxygen present inthe air helps in the process of combustion. Without oxygen, combustion will not take place.29. Can the process of rusting be called combustion? Give a reason for your answer.AnswerNo, because rusting is an exothermic process as heat is liberated during rusting. On the other hand,combustion is a chemical process in which a substance reacts with oxygen to release energy in theform of heat or light.ute30. Why are fires produced by burning oil not extinguished by pouring water?AnswerWater is heavier than oil. So when it is poured on an oil fire, it sinks to the bottom where itevaporates due to heat of the fire and thus expands rapidly due to which it may push and splatter firein all directions. Fire due to oil can be extinguished by using dry chemical fire extinguisher.tit31. Explain why, fire caused by electricity should not be extinguished by pouring water.AnswerInsThe most common fire extinguisher is water. But waterworks only when things like wood and paperare on fire. If electrical equipment is on fire, water may conduct electricity and harm those trying todouse the fire. For fires involving electrical equipment and inflammable materials like petrol, carbondioxide (CO2) is the best extinguisher. CO2, being heavier than oxygen, covers the fire like ablanket. Since the contact between the fuel and oxygen is cut off, the fire is controlled. The addedadvantage of CO2 is that in most cases it does not harm the electrical equipment.32. How is the fire caused by electricity extinguished?AnswershThe fire caused by electricity extinguished by petrol or by carbon dioxide. For fires involvingelectrical equipment and inflammable materials like petrol, carbon dioxide (CO2) is the bestextinguisher. CO2, being heavier than oxygen, covers the fire like a blanket. Since the contactbetween the fuel and oxygen is cut off, the fire is controlled. The added advantage of CO2 is that inmost cases it does not harm the electrical equipment.ka33. How is the fire produced by burning oil (or petrol) extinguished?AnswerAaThe fires caused by the burning of inflammable materials like oil or petrol are also extinguished byusing carbon dioxide fire extinguishers. Carbon dioxide used for extinguishing fire can be stored as aliquid at high pressure in cylinders.34. A drum full of kerosene catches fire. What is the simplest way to put off this fire?AnswerA small fire like a drum of kerosene on fire can be extinguished by throwing sand or soil over it.When sand is thrown over burning kerosene oil, the sand covers it like a blanket. The sand cuts offthe air supply to the burning kerosene oil due to which the fire gets extinguished.35. What is the first thing you should do if a fire caused by burning wood or paper?Answer

Water extinguishes heat by cooling down the burning substance such as wood and paper. Whenwater is thrown on these substances, it gets cooled below its ignition temperature and also stopsburning.36. (a) What does a Fire Brigade do when it arrives at a place where a building is on fire.(b) Describe one method of putting out a fire caused by burning wood or paper.Answerute(a) When fire brigade arrives to the rescue, the fireman throws a strong stream of water on thebuilding on fire, the burning material gets cooled down to below their ignition temperature and thefire is extinguished.(b) Water extinguishes heat by cooling down the burning substance such as wood and paper. Whenwater is thrown on these substances, it gets cooled below its ignition temperature and also stopsburning.tit37. Explain why, we are advised not to sleep in a room having closed doors and windows,with a coal fire burning inside.AnswerInsDue to the burning of coal, the available oxygen gets depleted and it leads to incomplete burning ofcoal. Incomplete combustion of coal gives carbon monoxide gas. It is a very poisonous gas. It isdangerous to burn coal in a closed room. The carbon monoxide gas produced can kill personssleeping in that closed room38. (a) What is a flame? What type of substance, on burning, gives a flame?(b) What is the difference between the burning of a candle and the burning of a fuel like coal?Answersh(a) A hot glowing body of ignited gas that is generated by something on fire is called Flame.Kerosene oil and molten wax are substances that give a flame while burning.(b) Substances which vapourize during burning produce flames, such as wax in the candle,Kerosene.kaSubstances which do not vapourize during burning do not produce flames. Example – coal andcharcoal.39. How does pouring water extinguished a fire?AnswerAaWater lowers the temperature of the burning substance. When the temperature goes down belowthe ignition temperature of the burning substance the fire extinguishes. The water here acts as acooling agent.40. Explain how carbon dioxide is able to control fires?AnswerCarbon dioxide is heavier than oxygen, covers the fire like a blanket. Since the contact between thefuel and oxygen is cut off, the fire is controlled. The added advantage of CO2 is that in most cases itdoes not harm the electrical equipment.41. If you see a person whose clothes are on fire, how will you extinguish the fire? Give areason for your answer.